Plant

A plant is an organism that belongs to the kingdom Plantae, which includes the extant divisions Green Algae and Land Plants. In common speech, the term will usually be used to recognize the latter group. Common features among plants include carrying out photosynthesis within chloroplasts (and the associated green color due to chlorophylls a and b), cellulose-based cell walls, and large water-filled central vacuoles within their cells, though there are occasional exceptions. There are a number of organisms, such as Kelp (brown algae) and Fungi, which were formerly considered plants, but are no longer classified as such due to morphological and genetic differences.

Including Liverworts, Hornworts, and Mosses, these plants lack vascular tissue and are therefore usually small, though often found in large clumps or carpets. They are primarily found in shady, damp areas, and spend most of their time haploid (having a single copy of each chromosome), though they develop a diploid structure upon fertilization called a sporophyte which produces haploid spores for reproduction.

Vascular plants are defined by their vasculature in the form of xylem, used to transport water & some nutrients upward, and phloem, used to transport organic nutrients (notably sugar) where it's needed. They include ferns and club mosses, which reproduce via spores, as well as the most populous division, seed plants. With the exception of club mosses, these plants demonstrate specialized stem, leaf, and root structures, which are what most people look for when seeking a plant.

Defined by the production of a specialized reproductive structure, the seed, this is the most populous group of land plants world wide. Their reproduction cycle involves the production of sperm-containing pollen, which is carried by wind or pollinators to other plants for fertilization. The best known types include conifers, which produce their seeds within cones, and the ever-popular flowering plants, which produce decorative organs specialized for attracting pollinators. Some flowering plants take this a step further and produce fruit, a mechanism used to attract animals for the purpose of spreading their seeds further. Curiously, certain species of mammal use flowers for the same purpose, as mentioned under "practical uses" below.

Despite some superficial similarities, Fungi are not plants. A large division is seen in their use of chitin, not cellulose, in their cell walls, as well as their general lack of chloroplasts (and therefore, photosynthesis). Interestingly, they are more closely related to animals than to plants.

Despite this, the USDA still classifies mushrooms in the Vegetable Food Group.[1]

Similarly, other forms of algae (excepting green) are not plants, despite carrying out photosynthesis. Where to draw the division between plants and algae has been a matter of some small debate; a common split is at the production of isolated reproductive structures, placing green algae with plants and most other forms (yellow, brown, red) in other groups by structure and genetics. To note, Cyanobacteria (blue-green algae) also undergo photosynthesis, and are likely related to the original chloroplasts[2].

Plants are sometimes described by pop-science writers as "natures chemical factories". They are essential to the food chain, as most plants can harness the energy from sunlight via photosynthesis. Photosynthesis is a process of extracting energy from an exogenic reaction between oxygen and carbon using light, with the result being the production of sugar, which is used both structurally and as an energy store. Green plants owe their green color to the chlorophyll contained in chloroplasts, in which this reaction takes place. In addition to carbohydrate sources, plants can make the many vitamins that cannot be made directly by animals.

Plants have other uses that aren't related to food. Woody trees can be used for construction, from solid hardwood such as oak to light and flexible bamboo. Further, wood is also used to produce paper, used today for writing upon, printing books, and showing festivity during parades and other celebrations. Flowers, produced by certain plants, are used to woo mates in the animal kingdom. And of course, a beautiful autumn foliage is purely for Man to gaze upon, thus proving that the earth is 6000 years old.[3]

↑Similar to mitochondria, chloroplasts are thought to have originated from endosymbiosis, in this case of Cyanobacteria. In layman's terms, a symbiosis formed in which early Cyanobacteria lived within early eukaryote cells, contributing energy & sugar, and the system eventually became fixed; they've been with plants and their relatives ever since.